I have same issue. Don't know why. For x=8 and given m=11110584286373380831 and n=908 number of combinations according to the formula is comb_num=11110584286373380096.0. Which is not equal (comb_num != m). Same for other x in range(n+1). Getting x=-1 when it is said that right answer is x=8. Using Python.
Guess it's because of precision issues.

I have this problem. The ones that work are passing, but the ones that should be impossible are producing a string.
I've already forfeited and looked at the solutions as I was spending way too long on this, but I can't for the life of me figure out why what I
did doesn't work even though it's extremely similar to the solution listed first.

Because you need to follow the steps, first get the smallest i, and then the smallest j.

Testing(209917, [29917, 0, 1]); 
                        ^
                      smallest i, 1 > 0, so [29917, 1, 0] is not valid

This is explained in the description, read it again.

testing(269045, [26945, 3, 0])

In that other case, to get the smallest number you can get moving only one digit, your only choice is to move the 0 to the front.